I'm guessing that most of those people have at least one account (on some sort of service) that uses the traditional email+password login. I don't think that ad

I'm guessing that most of those people have at least one account (on some sort of service) that uses the traditional email+password login. I don't think that adding one more thing (the server domain) to that is hard. Or they could just use the default (in basically every client it is matrix.org).